As the line width of semiconductor circuits is shrunk, stricter requirements for flatness of semiconductor wafers used as substrates of the circuits have been imposed. In such situations, large-diameter wafers are polished by a double-side polishing method, which ensures a higher polishing precision, instead of a conventional single-side polishing method.
Examples of the double-side polishing apparatus include a planetary gear type of double-side polishing apparatus, as depicted in FIG. 6, and a swinging type of double-side polishing apparatus, as depicted in FIG. 7. The planetary gear type of double-side polishing apparatus has a lower turn table and an upper turn table movable upward and downward, and is capable of pressing the upper turn table against the lower turn table to apply load to wafers interposed between the upper and lower turn tables. As depicted in FIG. 6, the double-side polishing apparatus 101 has a sun gear 107 provided inside the lower turntable and an internal gear 108 provided outside the lower turn table.
Furthermore, between the upper and lower turn tables, carriers 105 for holding wafers are provided, and each of the carriers 105 engages at its periphery the sun gear and the internal gear and thereby can rotate. The carrier rotates about its own axis and revolves with respect to the axis between the upper and lower turn tables according to the rotational speed of the sun gear and the internal gear. Wafers being polished are inserted and held in the respective holding holes 106 provided in the carrier are polished without flying out of the double-side polishing apparatus.
On the other hand, as depicted in FIG. 7, the swinging type of double-side polishing apparatus 111 has a carrier 105 for holding wafers provided between rotating upper and lower turn tables, and imparts a circular movement to the carrier 105, without rotating the carrier, to polish the wafers.
The planetary gear type of double-side polishing apparatus can rotate and revolve the carrier at a higher rotational speed and at a higher revolution speed than the swinging type of double-side polishing apparatus, and promote rotational movement of the wafers during polishing, thereby enabling the wafers to be polished with flatness higher than the swinging type of double-side polishing apparatus. The planetary gear type accordingly becomes recent mainstream of the double-side polishing apparatus.
Here, it has been known that, for the planetary gear type double-side polishing apparatus, the flatness of the polished wafer varies depending on the relationship between the thickness of a carrier and the thickness of the wafer at the end of polishing, that is, a finishing thickness of the wafer (See Patent Document 1, for example). For a finishing thickness thicker than the thickness of the carrier, for example, a wafer sinks into a polishing pad due to a polishing load so that pressure on the outer circumference portion of the wafer becomes higher than that on the central portion of the wafer. As a result, the higher pressure promotes polishing of the outer circumference portion, creating an outer circumference portion thinner than the central portion, that is, a so-called outer circumference sag, which tends to make the entire shape of the wafer convex.
Conversely, for a finishing thickness thinner than the thickness of the carrier, sinking a wafer into a polishing pad is mitigated (i.e., a retainer effect) so that pressure on the outer circumference portion becomes lower than that on the central portion. As a result, the outer circumference portion becomes thicker than the central portion, that is, a so-called outer circumference rise is created, which tends to make the entire shape of the wafer concave.
Accordingly, the flatness is conventionally adjusted by adjusting the finishing thickness of a wafer with respect to the thickness of a carrier. In general, a polishing process includes a first polishing in which rough polishing is performed at a high polishing rate and a second polishing in which fine polishing is performed at a low polishing rate, and the finishing thickness is adjusted by changing a polishing time of the first polishing. Polishing a wafer into a highly flat wafer needs an optimum finishing thickness with respect to the thickness of a carrier.
The finishing thickness of and the flatness of a polished wafer are measured with a flatness measuring device provided separately from a double-side polishing apparatus after the polished wafer is taken of the apparatus by an operator. After the measurement, the wafer is put back into the double-side polishing apparatus. The finishing thickness in a next polishing batch is determined to set the polishing time on the basis of the measurement result of the flatness. Since such measurement, involving taking out a wafer after every polishing batch, leads to reduction of apparatus's productivity and of operator's productivity, the measurement is performed every multiple polishing batches.